An 8-year-old Thoroughbred mare showing anestrus for longer than 6 months with left ovarian mass was referred to Veterinary Medical Teaching Hospital of Seoul National University. For 6 months, the mare had received 2 times hormonal therapies with $PGF_2{alpha}$ and progesterone. The mare had not responded to these therapies and not shown estrous behavior. On rectal palpation, the left ovary was enlarged and contralateral ovary was small and inactive. Transrectal ultrasonographic examination of the left ovary revealed a mass composed of multiple, irregularly shaped follicles. Granulosa cell tumor (GCT) was suspected according to her history. Ovariectomy was performed through the left flank laparotomy with a standing position in a simple chute located outside of horse farm. The mass was diagnosed as GCT on histopathology. The mare was recovered without any problem and became pregnant next spring.